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I personally think, there isn't any file handling limit however I believe it depends or effects CPU usage. We have event receivers on document libraries that are used by hundred of clients in a specific amount of time.

So in short, There isn't any "Known" limit however if a large number of events occur but your CPU/RAM isn't capable of handling processes then your SharePoint will most likely crash.

I never done it myself but you can use Visual Studio to test performance of your SharePoint portal or possibly event receivers.